1. Field:
This invention relates to a method for treating resinous coal, and more particularly to a method for separating fossil resin from coal by froth flotation.
2. State of the Art:
Certain bituminous coals of the Western United States are known to contain appreciable quantities of macroscopic fossil resin (resinite). Such resinous coals are found in the states of Arizona, Colorado, New Mexico, Utah, Washington, and Wyoming, etc. The Wasatch Plateau coal field in Utah has a particularly high content of fossil resin. It has been reported that some seams in this field average as much as 5% resin.
Fossil resins had been recovered intermittently from the Utah coal field since 1929 by gravity and/or flotation processes. The production, nevertheless, was on a very small scale and past separation technologies have limited the development of a viable fossil resin industry. Generally resin flotation concentrates must be refined by solvent extraction and evaporation of the solvent. Solvent-purified resins from the Utah coal field typically have a molecular weight of about 1000-1500, a melting point of about 170.degree. C. and an iodine number of about 145. This product, at the present time, has a market value of $0.50-0.70/lb as a chemical commodity and can be used in the ink, adhesive, rubber, varnish, enamel, paint and coatings, and thermoplastics industries. However, the technology for the recovery and utilization of fossil resins from coal did not receive sufficient research attention during the past decades. Research carried out in this field was very limited when compared to the research effort made for other energy and mineral commodities. Such a situation was due to the rapid development of petrochemical technologies after World War II and the abundance of synthetic resins. Because of the lack of technology and the competition from synthetic resins, this valuable fossil resin resource from western coal has been wasted for many years, being burned together with coal for electric power generation. Based on coal production data from the Utah region, it is estimated that at least 150 million pounds per annum of fossil resin from the Wasatch Plateau coal field is being used as fuel for electric power generation.
During the past decades, there have been a few patents issued which describe the flotation separation of fossil resin from coal by froth flotation. They include U.S. Pat. No. 1,773,997 (by Green, 1930), U.S. Pat. No. 1,869,532 (by Weining, 1932), U.S. Pat. No. 2,506,301 (by Klepetko, 1950), U.S. Pat. No. 2,591,830 (by Klepetko, 1952), U.S. Pat. No. 4,377,473 (by Laros and Pick, 1983), U.S. Pat. No. 4,724,071 (by Miller and Ye, 1988), U.S. Pat. No. 4,904,373 (by Miller, et al., 1990), as well as the USSR Patent 716,609 (1980). In all of these inventions, fossil resin particles are removed by attachment to dispersed air bubbles and the fossil resin particle/bubble aggregates float to the top of the flotation cell to form a froth product while coal and other gangue particles generally remain in suspension. In this way, fossil resin is separated from coal. The extent of the fossil resin separation from coal and the sophistication of the technology basically have progressed, from conventional flotation technology, to selective oxidation and selective dispersion. However, both fossil resin and coal exhibit a natural hydrophobicity and such separation is not easy.
Most work in the field of resin-coal separation by froth flotation techniques has realized higher alcohols (Laros; 6 to 8 carbon atoms alcohols), the use of amyl alcohol, a five carton alcohol, is disclosed in Green as a suitable frothing agent. Green used amyl alcohol as the sole frothing agent and indicated generally the use of higher alcohols, turpentine, cresol, pine oils and the like.
Recent important patents in this field are U.S. Pat. Nos. 4,724,071 and 4,904,373 issued in 1988 and 1990, to Miller, et al., respectively. U.S. Pat. No. 4,724,071 teaches the use of ozone to selectively oxidize the surfaces of finely-ground coal particles to achieve selective flotation separation of fossil resin from coal. This technology was invented based on the discovery that the native difference in hydrophobicity between the fossil resin and coal as measured by contact angle and bubble attachment time is small. Because of this fact, some of the coal particles in suspension always attach to air bubbles during the flotation of fossil resin, causing a poor separation efficiency. By selective oxidation with ozone, coal particles become extremely hydrophilic and will not attach to air bubbles during flotation while the natural hydrophobicity of fossil resin can be retained by proper control of ozone dosage and oxidation time. In this way, excellent flotation separations can be achieved with a flotation product containing as much as 95% resin at a recovery of 70-80%. However, environmental concerns, health/safety issues and cost associated with grinding have limited the adoption of this technology by the coal industry.
U.S. Pat. No. 4,904,373 teaches the importance of raising the suspension pH into the alkaline region so that the natural resin/coal particle aggregates in suspension can be effectively dispersed and the selective flotation of fossil resin can be achieved. This innovation was based on the discovery that fine coal particles tend to aggregate at the surfaces of fossil resin particles. Under these circumstances, the selective flotation of fossil resin from coal can be improved by any method which can effectively disperse the resin/coal particle aggregates and still maintain a suitable difference in hydrophobicity between the resin and coal particles. Although this pH control procedure results in a lower grade fossil concentrate than that given by selective ozone oxidation, it is expected to be most useful for industry in view of its simplicity, effectiveness and economy.
Nevertheless, the pH control technology, U.S. Pat. No. 4,904,373, which teaches how to achieve suitable dispersion for flotation of fossil resin from coal, is only one of the necessary conditions for selective flotation. In order to achieve selective flotation, another condition must be satisfied, namely the control of particle hydrophobicity. With conventional prior art reagents (frothers) some of the coal particles in the suspension are hydrophobic and tend to float together with resin particles. In this regard, necessary control has to be taken to minimize the flotation of coal particles in order to achieve the desired resin grade in the froth concentrate.
In view of the fact that a viable fossil resin industry has yet to be established, it is clear that improved separation technologies are needed for the more efficient recovery and separation of fossil resin from coal.